Difference between revisions of "Garnet"

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The composition of a garnet determines certain measurable characteristics of the stone such as [[refractive index]] and [[specific gravity]]. It can also influence the color of the stone, which is why garnets come in many colors and shades. The confusion which occurs from these mixed garnets, together with marketing strategies, have caused an array of trade names to be invented for various garnets. While it is scientifically correct to call the the green garnet found in Kenya a green grossular, the gem trade insists on (erroneously) calling it Tsavorite. It sells better that way. Some names are relatively new and are still being contested, others have been in use so long that they have gotten a firm grounding in literature and have been adopted by gemologists. It is important to remember that, at the end of the day, all garnets are a mix of two or more of the end members mentioned above.
 
The composition of a garnet determines certain measurable characteristics of the stone such as [[refractive index]] and [[specific gravity]]. It can also influence the color of the stone, which is why garnets come in many colors and shades. The confusion which occurs from these mixed garnets, together with marketing strategies, have caused an array of trade names to be invented for various garnets. While it is scientifically correct to call the the green garnet found in Kenya a green grossular, the gem trade insists on (erroneously) calling it Tsavorite. It sells better that way. Some names are relatively new and are still being contested, others have been in use so long that they have gotten a firm grounding in literature and have been adopted by gemologists. It is important to remember that, at the end of the day, all garnets are a mix of two or more of the end members mentioned above.
  
{| class="wikitable" style="margin-left:0; width:90%;"
+
{|class="wikitable" style="margin-left:0; width:100%;"
!colspan="2"|Garnet Names
+
|- bgcolor="#efefef" align="center"
 +
!colspan="3"|Common Garnet Names
 
|-
 
|-
|Rhodolite
+
|'''Almandine'''<br/>
|Anthill
+
Red with Violet Tint<br/><br/>
 +
'''Andradite'''<br/>
 +
Yellow, Green, Brown & Black
 +
*Demantoid<br/>
 +
Bright Green
 +
*Melanite<br/>
 +
Opaque Black
 +
*Topazolite<br/>
 +
Lemon-Yellow
 +
|'''Grossular'''<br/>
 +
Green, Yellow, Copper-Brown
 +
*Hessonite<br/>
 +
Brown-Orange
 +
*Tsavorite
 +
Intense Green
 +
*Leuco Garnet<br/>
 +
Colorless
 +
*Mali<br/>
 +
Yellow-Green to Yellow-Brown
 +
|'''Hydrogrossular'''<br/>
 +
Translucent to Opaque Green, Pink, White<br/><br/>
 +
'''Malaya'''<br/>
 +
Pinkish, Reddish or Yellowish Orange<br/><br/>
 +
'''Pyrope'''<br/>
 +
Dark Red to Reddish Orange
 +
*Anthill
 
|-
 
|-
|Malaya
+
|'''Rhodolite'''<br/>
|Hessonite
+
Purplish Red to Reddish Purple
 +
|'''Spessartine'''<br/>
 +
Yellowish Orange to Red-Brown
 +
*Mandarin<br/>
 +
Intense Orange-Yellow
 +
|'''Uvarovite'''<br/>
 +
Emerald Green
 
|-
 
|-
|Tsavorite
 
|Demantoid
 
|-
 
|Mandarin
 
|Topazolite
 
|-
 
|Mali
 
|Melanite
 
 
|}
 
|}
  

Revision as of 17:43, 3 January 2013

Meet the Family

Admired by mankind as far back as history can take us and available in many colors, garnets have been an important gem material throughout history. Research dates the use of garnets as a gem material back to the earliest known civilisations.

Pomegranate Seeds
This fruit may have given garnet its name.

The name garnet is derived from the Latin word for grain or seed. Historically the red varieties of this gemstone, which resemble the seeds of a pomegranate in color, were referred to as garnets. The term garnet has com to refer to the whole family of minerals that crystallize in the cubic system and share the same chemical blueprint. The elements used to fill that blueprint place a gem garnet into one or more of the following:

  • Pyrope: Mg3Al2(SO4)3 (magnesium aluminum silicate)
  • Almandine: Fe3Al2(SiO4)3 (iron aluminum silicate)
  • Spessartine: Mn3Al2 (SiO4)3 (manganese aluminium silicate)
  • Grossular: Ca3Al2(SiO4)3 (calcium aluminum silicate)
  • Hydrogrossular: Ca3Al2(SiO4)3-x(OH)4x (calcium aluminum silicate with hydroxide)
  • Andradite: Ca3Fe2(SiO4)3 (calcium iron silicate)

Garnet Names

It would have been easy if nature presented us with pure family members but, unfortunately for those trying to classify garnets, this is not the case. The family members listed above represent "end members" within the garnet group. In other words, their composition is an ideal, theoretical one. In reality, all the garnets we use as gem materials are a mix of two or more end members in various ratios. Some of the magnesium which makes a pyrope a pyrope can be replaced by iron, or some of the iron that makes an almandine an almandine is replaced by manganese. In gemology this 'mixing of end members' is called isomorphous replacement.

The composition of a garnet determines certain measurable characteristics of the stone such as refractive index and specific gravity. It can also influence the color of the stone, which is why garnets come in many colors and shades. The confusion which occurs from these mixed garnets, together with marketing strategies, have caused an array of trade names to be invented for various garnets. While it is scientifically correct to call the the green garnet found in Kenya a green grossular, the gem trade insists on (erroneously) calling it Tsavorite. It sells better that way. Some names are relatively new and are still being contested, others have been in use so long that they have gotten a firm grounding in literature and have been adopted by gemologists. It is important to remember that, at the end of the day, all garnets are a mix of two or more of the end members mentioned above.

Common Garnet Names
Almandine

Red with Violet Tint

Andradite
Yellow, Green, Brown & Black

  • Demantoid

Bright Green

  • Melanite

Opaque Black

  • Topazolite

Lemon-Yellow

Grossular

Green, Yellow, Copper-Brown

  • Hessonite

Brown-Orange

  • Tsavorite

Intense Green

  • Leuco Garnet

Colorless

  • Mali

Yellow-Green to Yellow-Brown

Hydrogrossular

Translucent to Opaque Green, Pink, White

Malaya
Pinkish, Reddish or Yellowish Orange

Pyrope
Dark Red to Reddish Orange

  • Anthill
Rhodolite

Purplish Red to Reddish Purple

Spessartine

Yellowish Orange to Red-Brown

  • Mandarin

Intense Orange-Yellow

Uvarovite

Emerald Green

Below is a diagram of trade names (developed by Dr. W. Wm. Hanneman) placed into context using their actual composition:

© W. Hanneman, used with permission

Garnets in History

Garnets have been used, wherever they occur, for as long as mankind can remember. From predynastic Egypt to the discovery and hype of Fanta orange garnets in Tanzania in 2007, garnets have never ceased to capture our attention. The Roman writer Pliny mentions gemstones called carbuncles, some of which are thought to be garnets. The red family member, almandine, was a very popular gemstone with the Germanic tribes which inhabited northern Europe during Roman times. They were used for cloisonné inlay work of which items found at Sutton Hoo are a great example.

Sut hoo2.jpg

During the Victorian era garnets were extremely fashionable; rose cut garnets from the Czech Republic covered entire jewelry items. These garnets are better known as Bohemian Garnets after the area they were mined: Bohemia.

Bohemian.jpg

Andradite was discovered in the Ural Mountains of Russia circa 1853. This distinctively bright green garnet was given the name demantoid in 1878. It quickly became a "hot" gemstone punctuating 'turn of the century' jewelry.

In the last quarter of the 20th century various additional garnet varieties have been found in Africa. These enjoy a great popularity to this day.

Gemological Properties

Gemological Information for Garnet
Color All Colors
Crystal Structure Cubic
Refractive Index 1.74-1.89
Durability Good
Hardness 6.6-7.5
Treatments Usually Not Treated
Country of origin Worldwide
Garnet Care
Ultrasonic cleaning Safe
Steam cleaning Safe
Warm soapy water Safe
Chemical attack Avoid
Light sensitivity Stable
Heat sensitivity Stable

Online G&G Articles on Garnet

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